Fatigue is a common, distressing, and disabling symptom across the lifespan and general population, but particularly in older adults and those with chronic illness. Although fatigue - perceived tiredness, lack of energy, or exhaustion - is often the focus of clinical care and research, it does not reflect how fatigue is experienced in the context of activity. Specifically, it is important to know how fatigued a person becomes as a result of engaging in different types of activities in order to fully understand their fatigue experience. For example, a person could report what looks to be a positive, low level of fatigue intensity. However their low fatigue could be due to the fact that they have severely restricted their lifestyle in order to avoid fatigue and would fatigue quickly if they attempted a more active lifestyle. In this scenario, the low fatigue score would mask the true severity of fatigue for the individual. Focus has recently turned to the concept of fatigability, or the level of mental or physical fatigue that results from activity, as a more comprehensive and informative measure of the fatigue experience. Two main types of fatigability have been identified: perceived fatigability (changes in self- reported fatigue in the context of activity) and performance fatigability (decrement in performance during activity). The emerging concept of fatigability has the potential to expand our understanding of the experience of fatigue and better evaluate the outcomes of clinical trials that are aimed at treating fatigue and fatigability. Unfortunately, the advancement of fatigability research is currently stymied by two major factors, the lack of a foundational conceptual model and inadequate measures of perceived fatigability. The goal of this study is to address these limitations through a two-phase study to develop a foundational conceptual model of fatigability and lay the ground work for a state-of-the-art self-report measure of fatigability, the Michigan Fatigability Inventory (MI-FI). We will conduct focus groups with normal aging adults, individuals with multiple sclerosis (MS), and individuals with fibromyalgia (FM) to develop a patient-centered conceptual model of fatigability (Specific Aim 1). Then, we will develop item pools for fatigability, the MI-FI, which is appropriate for use across the adult lifespan and individuals with medical conditions (Specific Aim 2). Item pool development will involve a multi-stage process including extensive literature review, development of items based on focus group content, cognitive interviews with normal aging adults and adults with MS or FM, translatability and literacy evaluation, and expert input. The patient-centered conceptual model produced in the proposed study will inform the development of other measures of fatigability (e.g., performance-based measures) and will provide a patient-centered focus to future research and treatment efforts. The item pools are a foundation for developing state-of-the art self-report measures of fatigability (the MI-FI) that will help to personalize clinical practice, provide tools to accelerate the testing of new therapies in clinical trials research, and significantly improve the lives of individuals who experience high fatigability.